This article is sort of a mess and I find it difficult to imagine anyone could learn about email from reading it. Too often it starts with something misleading and only clarifies later. For example, at first is says that recipients are given by headers in the message, but of course this is not the case. Recipients are part of the envelope. Envelope recipients are sometimes derived from the message, but sometimes not. I think it would be better to explain the SMTP envelope recipients first, which would lay the groundwork to explain that sometimes the recipients might be derived from RFC-822 header fields created by an MUA. But it's important to understand this might not always be true (there might not have been an MUA involved, or a webmail system with integrated mailer, like Gmail, might not need to put blind recipients in the message).